Multistrain experiments for screening toxic substances

Mouse models for radiation-induced cancers

Potential ionising radiation exposure scenarios are varied, but all bring risks beyond the simple issues of short-term survival. Whether accidentally exposed to a single, whole-body dose in an act of terrorism or purposefully exposed to fractionated doses as part of a therapeutic regimen, radiation exposure carries the consequence of elevated cancer risk. The long-term impact of both intentional and unintentional exposure could potentially be mitigated by treatments specifically developed to limit the mutations and precancerous replication that ensue in the wake of irradiation The development of such agents would undoubtedly require a substantial degree of in vitro testing, but in order to accurately recapitulate the complex process of radiation-induced carcinogenesis, well-understood animal models are necessary. Inbred strains of the laboratory mouse, Mus musculus, present the most logical choice due to the high number of molecular and physiological similarities they share with humans. Their small size, high rate of breeding and fully sequenced genome further increase its value for use in cancer research. This chapter will review relevant m. musculus inbred and F1 hybrid animals of radiation-induced myeloid leukemia, thymic lymphoma, breast and lung cancers. Method of cancer induction and associated molecular pathologies will also be described for each model.

Inbred Strains Should Replace Outbred Stocks in Toxicology, Safety Testing, and Drug Development

Methods of toxicity testing, barely changed for several decades, need to be improved. One way forward would be to use a small battery of inbred strains instead of the single outbred stock currently used in toxicity screening. Inbred strains are more stable, more uniform, more repeatable, and better defined than outbred stocks. Genetic variation would be observed as the difference between strains. Safety could be based on the most susceptible strain. Sometimes it may be possible to identify the genes involved. Mechanisms could be explored using gene expression profiling of susceptible and resistant strains. Two committees of toxicologists have concluded that the use of inbred strains, by controlling interindividual variability, would reduce the number of animals needed in toxicity screening, although both “preferred” outbred stocks. This preference appears to have been based on intuition rather than scientific principles. Data from a previously published study on the response to chloramphenicol in an outbred stock and four inbred strains is used to explain the advantages of the multistrain design. Toxicologists, safety pharmacologists, regulatory authorities, and pharmaceutical companies should take a critical look at the types of animals they use if they want to reduce the attrition rate of new drugs.

Improving toxicity screening and drug development by using genetically defined strains

According to the US Food and Drugs Administration (Food and Drug Administration (2004) Challenge and opportunity on the critical path to new medical products.) "The inability to better assess and predict product safety leads to failures during clinical development and, occasionally, after marketing". This increases the cost of new drugs as clinical trials are even more expensive than pre-clinical testing.One relatively easy way of improving toxicity testing is to improve the design of animal experiments. A fundamental principle when designing an experiment is to control all variables except the one of interest: the treatment. Toxicologist and pharmacologists have widely ignored this principle by using genetically heterogeneous "outbred" rats and mice, increasing the chance of false-negative results. By using isogenic (inbred or F1 hybrid, see Note 1) rats and mice instead of outbred stocks the signal/noise ratio and the power of the experiments can be increased at little extra cost whilst using no more animals. Moreover, the power of the experiment can be further increased by using more than one strain, as this reduces the chance of selecting one which is resistant to the test chemical. This can also be done without increasing the total number of animals by using a factorial experimental design, e.g. if the ten outbred animals per treatment group in a 28-day toxicity test were replaced by two animals of each of five strains (still ten animals per treatment group) selected to be as genetically diverse as possible, this would increase the signal/noise ratio and power of the experiment. This would allow safety to be assessed using the most sensitive strain.Toxicologists should also consider making more use of the mouse instead of the rat. They are less costly to maintain, use less test substance, there are many inbred and genetically modified strains, and it is easier to identify gene loci controlling variation in response to xenobiotics in this species.We demonstrate here the advantage of using several inbred strains in two parallel studies of the haematological response to chloramphenicol at six dose levels with CD-1 outbred, or using four inbred strains of mice. Toxicity to the white blood cell lineage was easily detected using the inbred strains but not using the outbred stock, clearly showing the advantage of using the multi-inbred strain approach.

The origins and uses of mouse outbred stocks

Outbred mouse stocks, often used in genetics, toxicology and pharmacology research, have been generated in rather haphazard ways. Understanding the characteristics of these stocks and their advantages and disadvantages is important for experimental design. In many studies these mice are used inappropriately, wasting animals' lives and resources on suboptimal experiments. Recently, however, researchers from the field of complex trait analysis have capitalized on the genetics of outbred stocks to refine the identification of quantitative trait loci. Here we assess the most widely used outbred stocks of mice and present guidelines for their use.

Bladder and liver tumorigenesis induced by 2-acetylaminofluorene in different F1 mouse hybrids: variation within genotypes and effects of using more than one genotype on risk assessment

Several F1 mouse hybrids were used in a chronic bioassay to determine whether such an experimental design would provide greater statistical power than using only the B6C3F1 hybrid. For this purpose, the dose response of formation of hepatocellular and bladder tumors after 30 mo of feeding 2-acetylaminofluorene (2-AAF) in the diet was assessed in 4 F1 mouse hybrids, including the B6C3F1 hybrid. No strain background-related differences in frequency of bladder neoplasms between any F1 hybrids were detected. Bladder tumors occurred only at the highest 2-AAF dose in female mice. In males the lowest dose was already sufficient to induce bladder neoplasms with incidences of 25-48% adjusted for different nontumor mortality patterns across doses. No marked strain-related differences in hepatocellular tumor rates were apparent in either sex. Higher frequencies of hepatocellular neoplasms were observed among the untreated control males of the B6C3, AY, and CVA F1 hybrids than among the comparable females. Among treated mice, the lowest 2-AAF dose increased liver tumor incidence, more so among the females than among the males. The different background genomes resulted in somewhat different risk assessments for liver tumor formation in males due to differences in the time-to-tumor curves. Except for the much higher background liver tumor rate in the CVY mice, the adjusted liver tumor incidences were similar across the four hybrids. Hence, the levels of statistical significance obtained for dose-response trends and comparisons of treated and control groups were similar using 48 animals per dose groups with B6C3 mice, or combinations of 24 animals per dose from 2 genotypes, or 12 animals per dose from the 4 hybrid genotypes. Estimates of carcinogenic potency for bladder tumors were similar, within a factor of two, across the four hybrids. However, estimates of liver tumor potency across genotypes varied by a factor of two and six for females and males, respectively. Thus, the mean of cancer potency estimates across these genotypes would be more representative for mice than results from any single genotype. As in chronic carcinogenesis studies with other test agents, neoplasms developed in only a certain proportion, rather than in all, of the genetically identical animals exposed to a given dose of the toxicant for the same length of time under the same controlled environmental conditions. This phenotypic variability in toxic responses may reflect differential regulation of gene expression among the genetically identical test animals.

Rat strain and stock comparisons using a functional observational battery: baseline values and effects of amitraz

A functional observational battery (FOB) was utilized to assess the effects of 3-day exposure to the formamidine pesticide amitraz in outbred Sprague-Dawley-derived and inbred Fischer-344-derived (F344) rats (both from Charles River Laboratories) and in outbred Long-Evans rats obtained from two commercial suppliers (Charles River Breeding Laboratories and Blue Spruce Farms). Significant strain and stock differences were obtained in baseline values for one-third of the FOB measures. In most cases, F344 rats were different from the others. Characteristic signs of amitraz exposure consisting of increased excitability, hyper-reactivity, and physiological and autonomic changes were evident in all treated rats. These effects increased with repeated dosing, and many were still present 6 days after dosing. On individual measures, there were differences between the strains and stocks in terms of sensitivity and time course of amitraz effects. In general, Blue Spruce Long-Evans rats displayed more effects of amitraz and F344 rats recovered more quickly than others. Although Sprague-Dawley rats showed the least effect overall, they displayed the largest increases in the sensorimotor responses to stimuli. These data indicate that although some behavioral and physiological parameters showed strain and supplier differences, in both baseline values and the effects of amitraz, conclusions concerning its neurotoxic potential in a screening context would be similar.